This paper examines the effects of using fuel particles of non-uniform size in packed bed combustion and gasification. Experiments were performed in an overfeed packed bed combustor fed with mixtures of different size fractions of charcoal particles. Gas compositions and temperatures within the bed as well as particle size analyses of the bed contents are presented for a number of different mixtures, covering a range of particle sizes from 6-24 mm. The experimental data are compared with a numerical model for packed bed combustion, and it is shown that the use of the volume-surface mean as an average feed particle size in the model gives good agreement with measured profiles of concentration, temperature, and average particle size in the bed. Other definitions of the mean particle size work nearly as well for gas concentrations and temperatures, but not for particle sizes within the bed. Mixtures with large differences in particle sizes may show some degree of segregation during feeding or sampling of the bed owing to the motion of small particles through the interstices between larger ones. (c) 2013 Elsevier Ltd. All rights reserved.